Method of bleaching paper pulp



Patented Oct. 11, 1927.

UNITED STATES 1,645,061 PATENT OFFICE.

OTTO KRESS, OF APPLETON, WISCONSIN, ASSIGNOR TO AMERICAN LAKES PAPER (10M- IPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF DELAWARE.

METHOD OE BLEACHING PAI'ER PULP.

No Drawing.

.The present invention relates to the bleaching of that kind of chemlcal wood pulp produced by the use of different alka= line cooking chemicals and commonly known to the trade as sulphate or kraft pulp. The chief' object of the invention is to produce a pulp for making paper, which, wh le possessing substantially the characteristic strength and toughness of ordinary sulphate or'kraft a er, shall at the same time have a far 'g ter and more attractive color.

Sulphate or kraft pulp, when bleached at all, is commonly bleached by means of the well known bleaching powder, or an equ1valent bleaching agent having chlorine as its active constituent. Usually such bleaching is only partial, since it is found 1n practice that such pulp cannot be more completely bleached, according to heretofore known methods, without suffering a very serious loss in the yield, a very decided decrease 1n strength of the finished product, and w thout the use of an excess of the bleaching agent. An excess of bleaching powder tends to destroy certain impurities left in the sulphate or kraft pulp, which impurities glve the essential characteristics of this kind of pulp and insure its strength andtoughness, and the excess of bleaching powder also ma- 80 terially diminishes the yield of the finished pulp. If sulphate pulp is cooked or treated with strong alkaline cooking chemical for a prolonged period and at hlgh temperature to produce the so-called easy bleaching pulp, its strength and yield are thereby re 1 duced and are further impaired by the subsequent bleaching, which still requires a relatively large amount of bleaching powder. Hence, with methods now in use, bleached sulphate pulp is expensive, of low yield and strength and is unsatisfactory in color.

I have discovered that by effecting a reliminary or partial bleaching of the sulpiiate or kraft pulp by bleaching powder or a recognized equivalent oxidizlng agent to a point at which such degree of bleaching is secured as will not materially impair the yield or strength of the pulp, I can thereafter, by the use of sodium bi-sulphite or an equivalent reducing bleaching chemical, complete the bleaching of the pulp so as to insure a far greater degree of whiteness, without impairing the strength or yield of the pulp or destroying the characteristics Application filed August 21, 1924. Serial No. 733,280.

which distinguish and give greatest value to kraft paper. I have further found that sulphate or kraft pulp that has riot been subjected to any preliminary oxidizing bleaching agent, can, by means of a reducing bleach such as sodium bi-sulphite, be partially bleached in a more satisfactory and economical manner than is now effected by the use of an oxidizing bleaching agent such as bleaching powder.

Iprefer to carry out my invention in the following manner:

Preliminary BZea0hz'ng.--The preliminary bleaching of the sulphate or kraft pulp is preferably (although not necessarily) effected by the so-called two-stage method; that is to say, two-thirds of the bleaching powder is added to the pulp to be bleached and on exhaustion of the bleach and after the washing out of the bleach residues, the final one-third of the total bleaching powder required is added, andi after it has been allowed to act, the pulp is washed by any of the well known methods to remove bleach residues and decomposition products. To prevent this preliminary bleaching from seriously impairing the strengthof the pulp, the use of acids, high temperatures and large amounts of bleaching powder should be avoided. From 300 to 600 pounds of bleaching powder .to a ton of dry pulp, dependent upon the desired degree of whiteness of the final product, may be employed, but, with the relatively higher amounts of bleaching powder, the two-stage method of bleaching should be used to avoid materially impairing the strength of the pulp. Fz'nal Bleaching-After the pulp has been preliminarily bleached, it may receive the final bleaching in either of the following ways :.-First-: The pulp while in a stock suspension of from 5 to 15% (i. e., 95-85% water and 515% stock), has added thereto preferably from 90 to 150 pounds of sodium bi-sulphite per ton of dry pulp, the stock being preferably agitated, to secure most effective action of the bleaching agent, for a period of at least 24 hours. After this stock as reached the proper degree of whitness, it may be put over any well known type of wet machine to separate the pulp and water and to produce laps,i. e., thick pulp which may run from say 30 to in bone dry content. Second: Preferably ,1 the final bleaching is accomplished as follows :The primarily bleached and washed pulp is run over a wet machine, and after the pulp lap or thick web has been formed, or is being formed on the bowl or winding roller of the wet machine, it is sprayed with a solution of sodium bi-sulphite. For this purpose, I

refer to use from 60 to pounds of sodium i-sulphite per ton of dry pulp, the approXimate strength of the spraying solution being 25 pounds of sodium bi-sulphite to 30 gallons of water, If the wet machine employed to separate the water from the pulp be of that type in which the pulp is not wound up or formed on the bowl of the machine, but is delivered from the machine in the form of a continuous web or sheet, the solution of sodium bi-sulphite may be sprayed onto the web as it leaves the machine.

Preferably after treatment with the sodium bi-sulphite in either of the Ways set forth, the pulp is stored in laps for a period of several days up to a maximum of a week, and will become brighter and lighter in color, since the sodium bi-sulphite is a reducing bleach and requires an appreciable length of time for its most effective action. The sodium lei-sulphite not only acts as a reducin bleach, but it I combines directly also wit certain groups, such as the aldehyde and ketone groups, present on the partially bleached pulp and produces colorless compounds. If desired, instead of storing the treated pulp in laps, it may be run from H the wet machine into storage tanks, allowed to remain for a suitable length of time (say from 18 to 24 hours) to enable the sodium bi-sulphite to complete the bleaching action, and, from the storage tanks, run direct to the heaters and then to the paper making machine.

The speed of the bleaching action of the bi-sulphite solution may be increased and a better color more readily produced, if a relatively small amount of zinc dust is added to the solution, preferably in about the proportion of 1 pound of zinc dust to 500 pounds of sodium bi-sulphite. The zinc dust reduces the sodium bi-sulphite and produces a more effective bleaching agent. Also, instead of the sodium bi-sulphite, I may use other reducing agents carrying an S0, radical, such as sodium sulphite, sodium hydro-sulphite, ordinary sulphite cooking liquor such as calcium bi-sulphite, 0r sulphurous acid; but the essential characteristic of the sodium bi-sulphite or its equivalent, is that it is a reducing bleach ,which combines with residual yellow color left in the pulp (after the preliminary treatment with bleaching powder) to form colorless compounds; and while the sodium bi-sulphite serves to reduce these yellow colored compounds and combines with them to produce colorless compounds, still it does not decrease the yield of the pulp nor'does it affect the strength of the pulp, as it does not attack the cellulose of the fiber itself in the pulp.

The improved process may also be modified by omitting the preliminary treatment with bleaching powder or other chlorine oxidizing bleach; the treatment being otherwise carried out as previously set forth. While the results obtained are not as satisfactory, if a very bright stock is desired, the caracteristic brown color of the orig'nal kraft pulp can be removed and a lighter colored stock produced. Such partially bleached stock may be satisfactorily employed for the manufacture of some grades of paper and particularly for certain colored papers in which the brightness of the shade is of importance.

I am aware that sodium bi-sulphite has been heretofore used in the bleaching'of ground wood or mechanical pulp, but so far as I am aware, no one prior to my present invention, has attempted to bleach chemical pulps, i. e., sulphate or kraft pulps with an oxidizing bleach to a point at which considerable of the impurities are removed but at which point there has been no material loss in yield or strength, and then finishing the bleaching operation by means of sodium lei-sulphite or equivalent reducing agent, to give the required whiteness to the pulp without impairing its strength or yield. It is also believed to--be quite new to bleach or partially bleach sulphate or kraft pulp with areducing agent such as sodium bi-sulphite. Obviously changes may be made in the details set forth without departure from the invention as defined in the claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent is 1. The method ofbleaching sulphate or kraft paper pulp which consists in subjecting the same first to the action of a chlorine bleaching agent and then to the action of a sulphite.

2. The method of bleaching sul hate or kraft paper pulp which consists in rst subjecting the pulp to the action of a preliminary oxidizing bleaching agent, subsequently subjecting this partially bleached pulp to the action of a sulphite to reduce and combine with the coloring compounds formed in the preliminary bleach, and then allowing the pulp to stand a sufficient time to effect the bleaching action of the sodium bi-sul hite.

3.The method of bleaching a sulp ate or kraft paper pulp which consists in subjecting this pulp to the action of a. sulphite and allowing the same to remain in contact with the pulp to reduce and combine with the coloring compounds existing in the pulp.

4. The method of bleaching sulphate or kraft paper pulp that consists in first subjecting the pulp to the action of bleaching powder, until the maximum light color is secured without materially reducing the strength or yield of the pulp, and subsequently subjecting this partially bleached pulp to the action of a bleaching agent to reduce the coloring compounds formed in the preliminary bleach.

5. The method of bleaching sulphate. or kraft pulp which consists in partially bleaching the same with a chlorine agent, 10

washing the partially bleached pulp'and treating the partially bleached and Washed pulp with sodium bi-sulphite.

. OTTO KRESS. 

